Cold cathode with nickel base, calcium oxide interface and magnesium oxide layer



March 22, 1966 B. FIRTH 3, 7

COLD CATHQDE WITH NICKE ASE, CALCIUM OXIDE INTERFACE AND MAGNESIUM OXIDELAYER Filed April 21. 1961 INVENTOR .fiimwmp 6- HA 7w BY 06 gm, WM mATTORNEY United States Patent COLD CATHODE WITH NICKEL BASE, CALCIUMOXIDE INTERFACE AND MAGNESIUM OXIDE LAYER Bernard G. Firth, Newark,N.J., assignor to Tung-Sol Electric Inc., a corporation of DelawareFiled Apr. 21, 1961, Ser. No. 104,601 1 Claim. (Cl. 313-346) Thisinvention relates to a cold cathode arrangement for vacuum tubes. Theinvention has particular reference to an intermediate layer whichreduces the voltage drop generally found in vacuum tubes having thistype of cathode.

Cold cathode tubes using primarily coatings of magnesium oxide, aluminumoxide, and mixtures of aluminum and beryllium oxide are known and havebeen used in circuits where the absence of a cathode heater is ofprimary importance. One of the disadvantages of these tubes lies in thefact that after the tube has been started and an anode-cathode currentestablished, the voltage drop existing between the metal cathode sleeveand the space adjoining the outer cathode surface is quite high, about200 volts. This cathode drop absorbs considerable energy and causesheating of the cathode material, thereby reducing the allowable maximumanode current which can safely be drawn through the tube. The presentinvention reduces this cathode drop by as much as 20% to 40%, andthereby increases the efficiency of the tube during normal operatingconditions.

One of the objects of this invention is to provide an improved coldcathode which avoids one or more of the disadvantages and limitations ofprior art arrangements.

Another object of the invention is to reduce the voltage drop in coldcathodes having primary coatings of magnesium oxide and other coatingshaving self-sustained electron emission.

Another object of the invention is to reduce the power expended whenstarting conduction in vacuum tubes having cold cathodes.

Another object of the invention is to increase the efficiency and thepower available in discharge devices using cold cathodes.

The invention comprises a cathode for electron discharge devices havinga base of conductive metal, an intermediate layer of metal oxide, and anouter coating of porous oxide. The intermediate coating, which may be acomposite layer of nickel and calcium or similar oxides, forms a lowwork function interface which permits easy passage of current from themetal base to the oxide coating. The intermediate layer may be formed bycoating the nickel sleeve with a dilute solution of a salt such as thenitrate of lithium, calcium, barium, strontium, etc., drying, and thensimultaneously oxidizing the nickel surface and salt by heating in dryair or in an oxidizing gas at 700 to 900 C. for a minute or so.

For a better understanding of the present invention, together with otherand further objects thereof, reference is made to the followingdescription taken in connection with the accompanying drawings.

FIG. 1 is a side view of an electron discharge device with parts cutaway to show the internal construction.

FIG. 2 is a vertical sectional view, greatly enlarged, of a portion ofthe cathode showing the metal base, the interface, and the outer oxidecoating.

FIG. 3 is a cross sectional view of the cathode shown in FIG. 2 and istaken along line 3-3 of that figure.

Referring now to the drawings, FIG. 1 shows a conventional vacuum tubehaving an evacuated envelope secured to a base 11 and having pins 12 forconnection 3,242,374 Patented Mar. 22, 19.66

to an external circuit. Inside the envelope 10 is an anode 13, asustaining electrode 14, and a cathode 15. The tube is exhausted to acondition Where the only means of conducting current between the anodeand cathode is the electron beam provided by the cathode surface. Thevacuum may be maintained by means of chemical getters if desired.

The electrodes within the envelope 10 may be supported in anyconventional manner, such as mica spacers 16. Lead-in conductors whichmake connection between the pins 12 and the electrodes within the tubemay pass through a reentrant stem 17. The view shown in FIG. 1 includesa cathode heater wire 18 within the cathode. This heater arrangement isnot used after the tube has been completed and is employed only duringthe manufacturing process to give the cathode a preliminary heating tobake out the occluded gasses. The heater wires 18 are generally notconnected to any of the pins 12 in the base. For a detailed descriptionof the operation of tubes containing magnesium oxide cold cathodes,reference is made to US. Patent 2,950,994 granted to Bernard G. Firth onAugust 30, 1960.

FIGS. 2 and 3 show the detailed construction of the cold cathode. Ametal base 20, which may be a hollow nickel tube, is the supportingmeans and connection is made to this part of the cathode for conductionto an external circuit. The interface layer 21 is composed of a mixtureof metal oxides such as nickel and barium oxide, calcium oxide,strontium oxide, lithium oxide, or similar low work function coatings.The outer coating 22 is the conventional magnesium oxide which ispreferably deposited by means of an electrostatic field which is appliedbetween the nickel base and a ribbon of burning magnesium. Other outercoatings such as aluminum oxide or beryllium oxide may be used.

To form the layer 21 it has been found convenient to first depositcertain salts of the metals onto the nickel surface, and heat thecathode in dry air or in an oxidizing gas so that the salt is reduced tothe metal oxide, giving off gas in the process. It has been found thatusing the nitrates or hydroxides of the metals produces good results.The salt is dissolved in a solution milligrams of calcium nitrate in 20cc. of distilled water), flowed onto the nickel sleeve, dried byspinning at 2000 r.p.m., and then heated to about 800 C. for about 1minute to form the oxide surface layer. Magnesium or other suitableoxide is then applied to this surface in any convenient manner. Whilethe electrostatic smoke application gives the best results, sprayingalso may be used. Calcium oxide is the preferred first coating.

The cathode is now mounted in the tube and processed in the usualmanner, see the above mentioned Patent 2,950,994. The resulting vacuumtube has a reduced cathode voltage drop, approximately half the cathodedrop found in prior tubes of this general classification. The tube iseasier to start, produces less heat, and has greater over-allefficiency. I

The tube shown in the drawings is a diode but it will be obvious thatthe cold cathode described herein may be employed in a triode amplifier,a pentode, or any other type of arrangement which uses a stream ofelectrons for conduction through an evacuated space.

The foregoing disclosure and drawings are merely illustrative of theprinciples of this invention and are not to be interpreted in a limitingsense. The only limitations are to be determined from the scope of theappended claim.

I claim:

A cold cathode for emitting electrons under the infiuence of an electricfield comprising, a nickel base for conductive connection to an externalcircuit, a first coating of calcium oxide to form an interface materialfor increasing conductance from the base to a second coat- 3 in'g, saidsecond coating including a layer of porous 2,950,993 magnesium oxide.2,960,659 3,007,075 References Cited by the Examiner 3,041,210 UNITEDSTATES PATENTS 5 3,156,844 12,302,127 8/1957 Dobischek et a1. 117 222 X2,840,751 6/ 1958 Meister et a1 313-346 X 2,925,511 2/1960 Skellett3'13-346 X 4 Umbreit 3 13-3 46 X Burton 313-346 X Schwender 313346 Mayer117-222 Veith 313-346 X GEORGE N. VVESTBY, Primary Examiner.

RALPH G. NILSON, ROBERT SEGAL, Examiners.

